In rats fed Crotalaria spectabilis, the ultrastructural features of the pulmonary artery will be followed and compared with the already described hypoxia-induced changes. Endothelial cell activity in both of these models of pulmonary hypertension will be assessed. The effect of intermittent hypoxia on the precursor smooth muscle cells of the peripheral pulmonary artery will be traced and the hemodynamic changes in such animals, with an indwelling pulmonary artery catheter, traced. We will follow the hemodynamic response of exercise and of acute hypoxia in rats with hypoxia-induced pulmonary hypertension and in rats that have been left to 'recover.' The effect of Verapamil on the 'in vivo' acute hypoxic response will be examined. Using an indwelling pulmonary artery catheter and aortic catheter, the effect of a corrected shunt and of a larger aorto-pulmonary shunt than previously used, will be studied in the growing pig. By banding the pulmonary artery, the effect of reduced flow will be assessed in the growing lung. Further cases and quantitative studies will be made on the lungs of neonates dying of Respiratory Distress Syndrome.